The heavy marble mask is located in the Cosmedin church in Rome. The original usage of the mask is unknown. In the 13th century, it was moved to the church where it is still located today.
The tradition is to put your hand into the mask's mouth and the legend says that if you are being insincere or not telling the truth, the mask will bite off your fingers. There is a scene in the iconic film Roman Holiday which depicts this tradition.
There are many replicas of the Mouth of the Truth scattered around the world (e.g. Las Vegas, San Francisco or Tokyo).
You can enter the church for free but for seeing the mask itself there is a €0.50 fee. Due to its popularity, there are often queues of tourists waiting to see the mask.
Free entry to the church, €0.50 donation for the Mouth of Truth.
Official Website http://www.turismoroma.it/cosa-fare/bocca-della-verita
Address Piazza della Bocca della Verità, Roma
Coordinates 41°53'17.724" N 12°28'53.558" E